4.19
Lights D. W. LEPORE R. A. WILLIAMSON
Types:
(1985)
B. G. LIPTÁK
(1995, 2005)
A. Incandescent B. Neon C. Solid-state (LED); Software is also available for virtual instruments, including virtual LEDs. Note: In the summary below the letters A to C refer to the above-listed indicator light types.
Operating Power Ranges:
A. 1 to 120 V AC, V DC; 8 mA to 10 amps B. 105 to 250 V AC, 90 to 135 V DC; 0.3 to 12 mA C. 1 to 5 V DC; 10 to 100 mA
Color of Unfiltered Light:
A. White B. Orange C. Red
Relative Brightness:
A. 1.0, 1.0 B. 1.0, 0.5 C. 2.0, 2.0
Average Useful Life:
A. up to 50,000 hours B. up to 25,000 hours C. up to 100,000 hours
Application Limitations:
A. Shock and vibration can cause early failures and generate considerable heat. B. Require high voltages and current-limiting resistors; have relatively low light output. C. Expensive; brightness is high but total light output is low.
Cost:
A heavy-duty, push-to-test, oil-tight pilot light for panel mounting costs about $75 to $100.
Partial List of Suppliers:
Allen-Bradley Co. (www.ab.com) AMP (www.amp.com) Automatic Switch Co. (www.ascovalve.com) Danaher Controls–Eagle Signal (www.danahercontrols.biz/eagsignal.htm) Eaton Corp. (www.eaton.com) GE Fanuc Automation (www.gefanuc.com) Hewlett-Packard Co. (www.hp.com) MicroSwitch, a Honeywell division (www.honeywell.com/sensing) Ronan Engineering Co. (www.ronan.com) Square-D Co. (www.squared.com) R. Stahl Inc. (www.rstahl.com) Westinghouse Process Control (www.westinghousepc.com)
INTRODUCTION Alarm lights are also discussed in Section 4.1, as status indicators, and in Section 4.18 as components in HMI (human–machine interface) systems. 812 © 2006 by Béla Lipták
Lighted indicator lights can convey several types of information to the operator. These include binary information, in which an on/off or open/closed condition can be displayed by a lighted (on) or unlighted (off) indicator, and status information, in which normal or alarm (abnormal) conditions are
4.19 Lights
813
dot array displays in red, green, or yellow colors, in 0.1- to 1.0-in. (2.50- to 25-mm) sizes. Such indicators are not discussed here because they have already been covered under “Digital Readouts and Graphic Displays” earlier in this chapter (Section 4.15). LIGHT SOURCE CHARACTERISTICS
FIG. 4.19a A seven-segment digital display, designed for good readability in brightly lit control centers. (Courtesy of Hewlett-Packard.)
expressed by legends, colors, and flashing or nonflashing indicators. Lights can be incandescent, neon, solid state, or softwarebased virtual displays. The amount of information that can be displayed by a single light is directly proportional to the equipment being monitored and its size or complexity. The use of redundant indicators is not recommended because their use reduces the attention value of all light indicators, and confusion can result if large numbers of them are used. Light-emitting diodes (LED) can not only be used as solid-state lamps, but also as bar segment (Figure 4.19a) or
Violet Blue
Red, redorange
Violet
Visible radiation Solid-state lamp (LED)
UV 100
IR
Incandescent lamp
Standard visi
on
80 Relative response
Yellow, yellowgreen
The most common types of lighted indicators are incandescent, neon, and solid-state lamps. Figure 4.19b shows their spectral response curves. The relative response ordinate at 100 corresponds to the peak sensitivity of the human eye and to the peak wavelengths emitted by the lamps. Standard vision, for example, is most sensitive at approximately 560 millimicrons, which is in the yellow and yellow– green band. The gallium arsenide phosphide (GaAsP) solid-state lamp peaks at a wavelength of 650 millimicrons, which is in the red and red–orange band. The curves in Figure 4.19b reflect the relative efficiencies of the light sources. The more efficient lights are the ones that have an output that nearly matches or falls within the standard vision curve. This output characteristic is approximated by both neon and the solid-state lamps. They are efficient because they convert most of their input power into light and emit little heat. While LED cluster-type lights are more expensive and their total light output is less than that of incandescent lights, their life expectancy is much longer (up to 100,000 hours) and their energy consumption is less because their operating
60
40 Neon lamp 20
0 300
400
500
600
700
800
900 9 Wavelength (millimicrons, 10 m)
FIG. 4.19b Spectral response curves of the human eye to various common lamp designs.
© 2006 by Béla Lipták
1000
1100
1200
814
Control Room Equipment
temperature is low. LEDs are also insensitive to vibration or shock and can be connected in an intrinsically safe manner. In addition, in the case of LEDs, defective bulbs can be identified (reduced luminescence) while the light is off.
1 1" 8 (29 mm)
1" − 1" 16 4 (1.5−6.5 mm) Panel thickness
13" 1 16
Light module
(30 mm)
Light Selection Important human factors in the selection and use of lighted indicators are visibility and arrangement. To transmit information, the indicator must be clearly visible to the operator. Variables that affect visibility are location, brightness, contrast, color, size, and whether the indicator is flashing or nonflashing. Critical indicators should be located within 30 degrees of the line of sight and should be at least twice as bright as the surface of the mounting panel. The use of dark panels is recommended because they furnish strong contrast to the indicator and reflect little light to the operator. When the ambient light levels are high, alarm legends should have dark characters imprinted on a light background. Inconsequential and routine messages should use the reverse combination. Colors and Flashing Colors can be powerful tools when properly used for lighted indicators. To avoid confusion, however, only a few colors should be used to code the different operating conditions. General information should be lighted in white, while normal conditions should be green. For abnormal conditions or in cases where caution is required, amber (yellow with a reddish tint) is a good choice because it affords maximum visibility. Red color should be used only for critical alarms that require immediate operator response. The use of blue or green lenses should be kept to a minimum. All lamps emit most strongly in the red and red–orange band. Consequently, much light is lost if it is filtered so as to appear blue or green. For important indicators, one should use the largest size that is compatible with the panel space. Flashing greatly improves visibility, but its use should be limited to critical alarms. The rate of flashing should be 3 to 10 flashes per second with the “on” time approximately equaling the “off” time. Light indicators should be arranged according to a functional format. Indicators associated with a manual control device (pushbutton or switch) should be placed right above the control device. It is best to locate related indicators on separate subpanels. Displays requiring sequential operator actions should be arranged in the normal reading pattern—from left to right or from top to bottom. Critical indicators should have dual lamp assemblies for additional reliability. A lamp check switch should be supplied to test for and to locate burned-out lamps for replacement. Lenses and Operating Environments The selected lenses should be diffusive and should eliminate glare or hot spots. The lens should also provide a wide angle of view (120 degrees minimum), and if side visibility is
© 2006 by Béla Lipták
9" 32 (32 mm) 1
Standard pilot light
15" 16 (75 mm) 2
Push-to-test pilot light
FIG. 4.19c Pilot lights for panel mounting. (Courtesy of Square-D Co.)
required, it should protrude over the mounting surface. The lens must be large enough to accommodate the required legends. Legends are commonly produced by hot stamping, engraving, or photographic reproduction of transparencies. Ordinarily, hot stamping is the most economical, whereas photo-transparencies furnish the sharpest characters and are the most versatile. Environmental parameters also affect the operation of indicator lights. Special designs are available for shock, vibration, or high-temperature applications. Rapid dissipation of heat is important if the indicator generates heat. Dripproof or watertight designs should be selected if the indicators are to operate in high humidity or corrosive atmospheres or if the lights will be placed on panels that are periodically washed down. Figure 4.19c illustrates some heavy-duty oil-tight pilot lights designed for panel mounting applications. They are available in both standard and push-to-test versions. Light Components The main components of most indicator light assemblies (Figure 4.19d) include the lamp-holder, the lamp, and the lens. Panel light types are usually secured to a panel by a nut and a lock-washer. Cartridge models can be held in place by a speed-nut type friction clip. Snap-in lights are usually retained by expandable latching fingers. Power can be supplied through wire leads and solder, screw, or quick-connect terminals. The heart of all lighted indicators is the lamp or light source itself. The three types (Figure 4.19e) of lamps in common use are incandescent, neon, and solid-state designs. The major parts of a lamp are the bulb (containing the light emitter) and the base. Lamps are also classified according to bulb shape and size and by the type of base. Bulb shape and size are designated by a letter that describes the shape and by a number that gives the nominal diameter in eighths of an inch. For example, a T-1 lamp has a tubularshaped bulb that is one eighth of an inch in diameter. Common bases are bayonet, screw, flanged, grooved, and bi-pin. Some
4.19 Lights
Lens Lens Neon lamp Lamp
Lampholder Resistor
Lampholder Wire leads Power terminals Cartridge light
Pilot light
Lens Latching fingers Lampholder
Terminals Snap-in-light
FIG. 4.19d Indicator light assemblies.
lamps have no base at all and are only supplied with wire terminals.
815
one third of the radiation emitted from this lamp is in the visible band (white light), while the rest is in the infrared band (heat). This means that approximately two thirds of the input power is emitted as heat. If large quantities of incandescent lamps are to be operated continuously and mounted closely together, special allowance for adequate heat dissipation is necessary. If the lamp is likely to experience shock and vibration, a low-voltage, high-current design should be used because it has stronger filaments. Lamps of 6 volts or less usually have short, thick filaments, whereas lamps of more than 6 volts generally have longer and thinner filaments. In all cases, however, the lamp should be tested under simulated operating conditions. Incandescent lamps can operate from 1 to 120 volts AC or DC. Current drain will be from 10 milliamperes to 10 amperes. Figure 4.19f shows the relationship of the applied voltage, lamp life, current, and light output. Variations in applied voltage have a drastic effect on lamp life. It is common practice to improve life and sacrifice some light by operating the lamp at slightly below rated voltage. For example, a 6-volt lamp operated at 5 volts will have roughly eight times the normal life and will still provide 60% of normal light. Over-voltage results in nearly the opposite effect: A 6-volt lamp operated at 7 volts will have only about one sixth of its normal life and will provide one and one half times the normal light. Therefore, controlling applied voltage is very important. Space permitting, a large lamp is preferred to a small one because its cost will be lower and its life and reliability higher. The larger lamp will also emit less heat than the smaller one of equal light output because the former has more surface area and its filament operates at a lower temperature.
LAMP TYPES Incandescent Lamps
Neon Lamps
The incandescent lamp shown in Figure 4.19e consists of a coiled tungsten filament mounted on two support wires in an evacuated glass envelope. When current is passed through the filament, its resistance causes it to glow and to emit both light and heat. In Figure 4.19b it can be seen that only about
The neon lamp shown in Figure 4.19e consists of two closely spaced electrodes mounted in a glass envelope filled with neon gas. When sufficient voltage is applied across the electrodes, the gas ionizes, conducts a current, and emits light and heat. All neon lamps require a current-limiting resistor in series with the lamp to guarantee the designed life and light characteristics, an example of which is the cartridge light in Figure 4.19d. The orange light emitted by this lamp is easily seen because a large portion of it falls within the standard vision curve (see Figure 4.19b). Since most of its emitted radiation is in the visible band, little heat is emitted. An important consideration, however, is that although the neon lamp is an efficient light source, its total light output is low. A clear or lightly diffusing lens should be used with it so that only a small portion of the light is absorbed. Neon lamps are very satisfactory for use under conditions of severe shock and vibration. The rugged mechanical construction avoids the use of the fragile filament of the incandescent lamp. Neon lamps will operate only on high voltages
PN Junction semiconductor Bulb
Bulb Filament
Lens
Electrodes Case
Base
Base Leads
Incandescent lamp
FIG. 4.19e Common lamps.
© 2006 by Béla Lipták
Neon lamp
Solid-state lamp (LED)
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Control Room Equipment
150
10
13
150
140 Percent of rated voltage
16
2000 1500 1000 700 525 400 325 250 200 160 125 100 85 75 65 55 48 42 36 32 28 26 22 19
Percent of life
Life (top scale) and Light (bottom scale)
130
140 130
120
120
110
110
100
100
90
90 Current
80
80
70 40
70 50
60
70
80
90 100 110 120 130 140 150 160 170 180 190 200 Percent of current and light output
FIG. 4.19f
Incandescent lamp characteristics.
and commonly run directly from standard 120 or 240 volt AC line voltages. Because of the high voltage, they require little current and power.
can use with their Visual Basic programs. The virtual LED displays can mimic the functions of stand-alone LEDs.
Solid-State Lamps (LEDs)
CHECKLIST
The solid-state lamp shown in Figure 4.19e is commonly called a light-emitting diode (LED), and it is a valuable byproduct of semiconductor technology. It is basically a P–N junction diode mounted in a hermetically sealed case with a lens opening at one end. Light is produced at the junction of P and N materials by two steps. First, a low-voltage DC source increases the energy level of electrons on one side of the junction. In order to maintain equilibrium, the electrons must return to their original state. Therefore, they cross the junction and give off their excess energy as light and heat. The light output of the popular gallium arsenide phosphide LED (see Figure 4.19b) has a very narrow bandwidth centered in the red band. These lamps are very efficient and have an exceptionally long life but are small and have low light output. The electrical characteristics of the LED are similar to those of the silicon diode. They are compatible with integrated circuits and operate directly from low-level logic circuits. Solid-state lamps, like neon lamps, perform satisfactorily under shock and vibration.
1. Determine operating voltage. 2. Select lamp type and size. 3. Select lens for type, color, size, and shape. The last two features should be large enough to hold the necessary legends. 4. Select a lamp holder that is compatible with both lamp and lens. Also consider the allowable panel space and the methods of mounting and providing electrical connections. 5. Test the indicator under simulated operating conditions.
Virtual Lights Virtual displays can be programmed to resemble all the familiar analog instruments, including running lights. The Microsoft Visual Basic programming language is popular in depicting virtual instruments. This language is popular because it is easy for third-party vendors to develop products that their customers
© 2006 by Béla Lipták
CONCLUSIONS The possible uses of indicator lights to display information are limited only by the designer’s imagination. There is usually one particular combination of lamp, lens, and lamp holder that is best suited for an application. Incandescent lamps are preferred for most applications because they are available in the widest range of light output, sizes, and voltages. The low light output of both neon and solid-state lamps limits their use to on/off indicators. Amber lenses should be widely used because they absorb little lamp light and because amber is the most visible of all colors. Snap-in lamp-holders should be used wherever possible because they require no mounting hardware and take little assembly time.
4.19 Lights
Bibliography Bylander, F. G., Electronic Displays, New York: McGraw-Hill Book Co., 1979. Chichibu, S. F. (Ed.), Nitride Semiconductor Blue Lasers and Light Emitting Diodes, London: Taylor & Francis, 2000. Degani, A., et al., “Modes in Human–Machine Systems,” Human Factors Research and Technology Web page, NASA, http://human-factors. arc.nasa.gov, 2001. Gadberry, B. E., “Designing Integrated Control System Displays,” Paper #91-0354, 1991 ISA Conference, Anaheim, October 1991. Jutila, J. M., “Guide to Selecting Alarms and Annunciators,” InTech, March 1981. McCready, A. U., “Man–Machine Interfacing for the Process Industries,” InTech, March 1982.
© 2006 by Béla Lipták
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National Electrical Code, Article 410, National Fire Protection Association, 1981. NUREG/CR-6633, “Advanced Information Systems Design,” U.S. Nuclear Regulatory Commission (USNRC), 2000. Schubert, E. F., Light-Emitting Diodes, Cambridge: Cambridge University Press, 2003. Shirley, R. S., et al., “What’s Needed at the Man/Machine Interface?” InTech, March 1981. Technical Report 1001066, “Human Factors Guidance for Digital I&C Systems and Hybrid Control Rooms,” Palo Alto, CA, Electric Power Research Institute (EPRI), 2000. Zukauskas A., et al., Introduction to Solid-State Lighting, New York: WileyInterscience, April 2002.
